1. Field of the Invention
The field of art to which the claimed invention pertains is solid bed adsorptive separation. More specifically, the claimed invention relates to a process for the separation of water from a feed mixture comprising ethanol and water which process employs a solid adsorbent which selectively removes the water from the feed mixture.
2. Description of the Prior Art
Diminishing world supplies and availability of crude oil as well as sporadic regional shortfalls of gasoline for motor fuel have created considerable incentive for the development and use of alternative fuels. Ethanol is gaining wide popularity as such a fuel, particularly when mixed with gasoline to form a mixture known as gasohol. Gasohol may contain up to about 10 vol. % ethanol, without modifications to presently used automobile engines being required, thereby extending the volume of motor fuel availability by a like percentage.
The primary source of the ethanol used in gasohol is derived primarily from the fermentation of mash, usually from corn. Natural fermentation is able to produce an ethanol-water product mixture containing at the most about 12 vol. % ethanol. This mixture may easily be concentrated by distillation to about 95% ethanol. Higher concentrations of ethanol, however, as required in gasohol are obtained only by expenditures of great amounts of energy and great difficulty due to the formation of an ethanol-water azeotrope at about the 95% ethanol concentration. A means of achieving the greater than 95% ethanol concentration without such a great expenditure of energy would thus be extremely valuable. One, therefore, might consider the many known adsorptive separation processes known in the art for possible application to the separation of ethanol from water.
For example, it is well-known in the separation art that certain crystalline aluminosilicates can be used to separate hydrocarbon species from mixtures thereof. The separation of normal paraffins from branched chain paraffins, for example, can be accomplished by using a type A zeolite which has pore openings from 3 to about 5 Angstroms. Such a separation process is disclosed in U.S. Pat. Nos. 2,985,589 and 3,201,491. These adsorbents allow a separation based on the physical size differences in the molecules by allowing the smaller or normal hydrocarbons to be passed into the cavities within the zeolitic adsorbent, while excluding the larger or branched chain molecules.
U.S. Pat. Nos. 3,265,750 and 3,510,423, for example, disclose processes in which large pore diameter zeolites such as the type X or type Y structured zeolites can be used to separate olefinic hydrocarbons.
In addition to separating hydrocarbon types, the type X or type Y zeolites have also been employed in processes to separate individual hydrocarbon isomers. In the process described in U.S. Pat. No. 3,114,782, for example, a particular zeolite is used as an adsorbent to separate alkyl-trisubstituted benzene; and in U.S. Pat. No. 3,668,267 a particular zeolite is used to separate specific alkyl-substituted naphthalenes. In processes described in U.S. Pat. Nos. 3,558,732, 3,686,342 and 3,997,620, adsorbents comprising particular zeolites are used to separate para-xylene from feed mixtures comprising para-xylene over the other xylene isomers. In the last mentioned processes the adsorbents used are para-xylene selective; para-xylene is selectively adsorbed and recovered as an extract component while the rest of the xylenes and ethylbenzenes are all relatively unadsorbed with respect to para-xylene and are recovered as raffinate components. Also, in the last mentioned processes the adsorption and desorption may be continuously carried out in a simulated moving bed countercurrent flow system, the operating principles and sequence of which are described in U.S. Pat. No. 2,985,589.
Any of the above adsorbents of the above processes might be effective for the separation of water from ethanol since all of those adsorbents are hydrophilic, i.e. they would probably be selective for water over the ethanol. However, there would be the problem of what could be used as an effective desorbent. The separation of the desorbent, if possible, from the ethanol raffinate and water extract would be considerably more costly than the primary distillation of the alcohol from the water.
We have discovered an adsorbent selective for water over ethanol which may be used in a process not requiring a desorption step and which is itself a source of ethanol.